road and workshop
Page 43
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by Handyman
Benchwise: lathe sense (13)
THE use of the single-point boring tool as a means of dealing with straight-bush work where the tool goes right through has already been discussed, and now I am going to deal with other tasks where this tool can usefully be used—holes in castings, brackets, etc, that do not go right through. These are blind holes, and may have to carry a bush or bearing-track and need to be bored and finished at a precise diameter and depth.
It may not at first be apparent to the motor mechanic just where this work will be required on modern vehicle repair, but I can assure him that while many tasks are quite impossible without a lathe, often those same tasks are not given a thought even where one is available simply because they bear no resemblance to lathe jobs.
In this industry we often have a crash repair, particularly of engines, where frontal impact has smashed or fractured timing covers, pump housings, compressors, generators, water pumps, etc, and all too frequently the only answer appears to be a lengthy wait for a new part. Yet with low-temperature welding available . today, and the new methods of placing hot or molten metal just where it is wanted with almost nil expansion risks, far more damaged parts are now within the scope of the repairer, and I do not necessarily mean the outside specialist.
It pays any fair-sized vehicle operator
with his own maintenance section to appraise both the welding and turning aspects of garage work, in relation to his own needs, his geographical position, and the availability or otherwise of suitable labour: where this latter aspect is favourable he can safely "soup up" both trades with an assurance of ample outside work.
Both these reclamation sections can be fully employed on work devoted to cutting vehicle downtime, and if his operatives are good he will have no trouble at all in selling any of their spare time profitably on the open market. While it will be interesting at a later date to look at new metal repair techniques, I must now return to the lathe and boring out these blind holes. Earlier I mentioned timing covers, etc, items that suffer in any frontal impact, and as they are not exactly fast-moving parts, the wait for a replacement can be a long and expensive matter, However, when we are faced with a damaged cover that can be put together by a good welder, this will mean that one or more bearing apertures will need to be built up partly, or filled in, needing true-boring attention.
Obviously, one cannot just go ahead and let the welder do his stuff, and then hope to pinpoint boring centres later—these must be determined accurately from the damaged parts. The sensible approach to this is to make a pattern or template of the cover face that will clearly indicate both fixing-holes, and the apertures themselves. This can be cut from a steel sheet about .020in. thick, and its accuracy checked by offering it to the engine casing with the bearings mounted on their respective shafts or stub spindles.
Having settled the pattern problem with the casing returned to the welder, the next task is to install the face plate, as in this instance the work must revolve. As we are not boring right through, the outer face of the casing can be set back to the face plate as snugly as possible, and while the uneven face of the casing does not make this easy, at least two true flat surfaces should be sought, such as the crankshaft aperture face and a bolt-hole boss. If there is only one, the crankshaft opening, and this is recessed below the general face of the casing, then resort must be made to these items I mentioned earlier, our old bearing-track packings. Choose the most suitable for the opening, and let this become the true foundation for the job.
Other location points must be arranged, as the casing cannot sit on one remote point and take a turning lead some distance away unsupported; here we use two or three other bolt-holes available and set into them good-fitting, long-threaded bolts with double nuts, which can go through the face plate slots, or butt to the face as needed. Once rigged and secured, a dial-gauge will
indicate the running accuracy of the casing face and any inaccuracy can be removed by bolt adjustment.
It is now necessary to make certain of the security of the casing to the plate, then it can be eased and moved across the face plate to line up for the first bore hole; the bearing holes in the template will, of course, have been cut a push-fit over the actual bearing. The final task after the template is secured to the casing face and the work has been truly centred through the template hole is to again check the true run of the casing face and finally to tighten and also mark off on the lathe tool the exact depth of the required bore hole, and set the lathe for slow-speed fine-feed and cut. The hole should now be bored to within a few "thous" of final depth and diameter.
Then the turner should concentrate on depth only, taking light cuts along the bottom of the hole until at the correct depth. A trick can now be applied that is as old as lathe work—this is to let the tool run into the wall of the bore at the base of the hole for three or four "thous"—this makes a slight Undercut right in the bottom corner. No effect will be made on the bearing track that will sit there later, as all bearings are rounded at their rim edges at least a similar amount. Now the final "fit" cuts can be taken to the diameter required to permit the bearing to be a firm push or press-fit on assembly. As the final cuts are made, and as the tool reaches the bottom, the operator will clearly hear the tool go "off-cut" as it reaches the undercut area. This trick has many applications, and is a reliable guide, well worth the trouble of preparation when the last few "thous" of a blind bore-hole are critical.